Polarization modulators find applications in such diverse areas as fiber optics communication, welding goggles, and time-multiplexed stereoscopic 3D displays. Liquid crystal cells are particularly well suited for modulating the states of polarization of light passing through them because the liquid crystal material itself is birefringent and the optic axis direction of this birefringent material can be controlled with an applied voltage. For some applications, a polarization modulator is used as a polarization switch, which switches light from one polarization state to another. To achieve the highest performance in time-multiplexed stereoscopic 3D applications, it is desirable to switch between two orthogonally related polarization states, such as between right-handed circularly polarized light and left-handed circularly polarized light or between vertically polarized light and horizontally polarized light.
There are two basic technologies used for time-multiplexed stereoscopic 3D systems, in which the left eye and right eye images are presented frame sequentially by an imaging device. One of the basic technologies entails use of active viewing glasses worn by an observer. Each eyepiece of the active glasses is equipped with a lens assembly comprising a polarization switch positioned between two polarizing films. The active glasses and imaging device operate in synchronism, and each lens assembly alternately passes to and blocks from its associated observer's eye images sequentially presented during alternate subframes of substantially equal duration so that the right eye images and the left eye images reach, respectively, the observer's right eye and the observer's left eye. The other basic technology entails use of passive viewing glasses worn by an observer and placement of a polarizer and a polarization switch in front of the imaging device. The polarization switch and imaging device operate in synchronism so that left eye images and right eye images propagate through a transmission medium while in different polarization states imparted by the polarization switch. Each eyepiece of the passive glasses is equipped with a lens comprising a polarizing film oriented to analyze the states of polarization of incident light carrying the left and right eye images to alternately block and pass them so that the right eye images and the left eye images reach, respectively, the observer's right eye and the observer's left eye. The present disclosure relates to the stereoscopic 3D technologies that use either active or passive viewing glasses.
One of the first polarization modulators using liquid crystals was the twisted nematic (TN) cell. The TN cell, taught by Helfrich and Schadt in Swiss Patent No. CH532261, consists of liquid crystal material of positive dielectric anisotropy sandwiched between two substrate plates having optically transparent electrodes whose surfaces have been processed to orient at right angles the directors of liquid crystal material contacting one surface relative to the orientation of the directors of liquid crystal material contacting the other surface. In the absence of an applied voltage, the liquid crystal directors inside the liquid crystal device uniformly twist 90° from the inside surface of the bottom substrate to the inside surface of the top substrate. This has the effect of rotating linearly polarized incoming light by 90° through a “waveguiding” principle. Upon application of a voltage to the liquid crystal device, the liquid crystal directors align perpendicular to the substrate, with the result that the twisted liquid crystal director structure disappears and with it the ability to rotate the linearly polarized incoming light. Thus, the TN cell can be considered as a polarization switch that rotates the direction of linearly polarized light by 90° when no voltage is applied and does not rotate the linearly polarized light when a sufficiently high voltage is applied. A problem with using a TN device as a polarization switch is that the transition from a high voltage optical state to a low voltage optical state is too slow for many applications because the restoring torque on the liquid crystal directors comes only from elastic forces propagating from the fixed boundary alignment established by the directors contacting the processed inner surfaces of the electrodes. This is referred to as an unpowered transition. The transition from a low voltage optical state to high voltage optical state, on the other hand, can be very fast because the torque on the molecules now comes from the coupling of the applied electric field with the induced dipole moment of the liquid crystal material. This is a powered transition. Even with low viscosity, high birefringence liquid crystal materials and the liquid crystal display device technology now available, the high voltage optical state to low voltage optical state transition is still on the order of 2 ms to 3 ms, which is too slow for use in modern time-multiplexed stereoscopic 3D applications, in which complete left or right eye images might be available for only 4 ms or less.
Freiser in U.S. Pat. No. 3,857,629 describes a TN polarization switch in which switching from low to high voltage optical states and from high to low voltage optical states are both powered transitions and thus both can be very fast. This switching scheme uses a special “two-frequency” liquid crystal mixture, the dielectric anisotropy of which changes sign from positive to negative for increasing drive frequencies. Applying a DC or a low frequency AC voltage powers the TN device on, and applying a high frequency AC voltage powers the TN device back off. There are, however, several problems associated with the two frequency technology. First, this scheme is incapable of switching uniformly over a large area because of formation of domains or patches in the liquid crystal device. Second, the crossover frequency, i.e., the frequency at which the dielectric anisotropy of the liquid crystal changes sign, is very temperature dependent and as a consequence limits the temperature range in which the device can successfully operate. Third, the high frequency drive signal feeding into the capacitive load of the liquid crystal device requires significant power, which precludes using this system in battery operated, portable devices such as active stereoscopic 3D glasses.
Bos in U.S. Pat. No. 4,566,758 describes a liquid crystal-based polarization switch operating in an electro-optical mode. The liquid crystal device described by Bos has become known as the pi-cell. The pi-cell polarization switch can rotate the polarization direction of linearly polarized light by 90°, but its operation is based on a switchable half-wave retarder rather than the 90° “waveguiding” principle of the TN display. This pi-cell mode switches faster than does the TN mode because the internal liquid crystal material flow associated with switching of the pi-cell does not introduce a slowing “optical bounce.” Nevertheless, the high voltage optical state to low voltage optical state transition is still an unpowered transition, with a response time of about 1 ms using present materials and device technology. Even a 1 ms response can introduce image crosstalk, loss of brightness, and other artifacts in modern time-multiplexed stereoscopic 3D applications.
Clark and Lagerwall in U.S. Pat. No. 4,563,059 describe a liquid crystal polarization switch based on ferroelectric liquid crystal materials, which belong to a different liquid crystal class from that of nematic liquid crystal materials described above. The class of ferroelectric liquid crystals differs from the class of nematic liquid crystals in that the ferroelectric liquid crystal molecules arrange themselves in layers. A ferroelectric polarization switch can very rapidly switch back and forth between two polarization states because both optical state transitions are powered transitions. However, there are many drawbacks of ferroelectric polarization modulators. First, the liquid crystal device is required to have a very thin cell gap, on the order of 1 μm, which makes it difficult to manufacture ferroelectric liquid crystal devices with high yield. Second, the alignment of the ferroelectric layers is very sensitive to shock and pressure variations, which sensitivity rules out many applications that entail manipulation, such as use in active stereoscopic 3D glasses worn by an observer. Third, variations in temperature can also cause alignment disruptions, especially if the temperature is temporarily raised above the smectic transition temperature.
Other polarization switches use two liquid crystal devices arranged in optical series. Bos in U.S. Pat. No. 4,635,051 describes a light gate system comprising first and second variable optical retarders, in which the projections of their optic axes on the light communicating surfaces of the variable retarders are orthogonal and which are placed between crossed polarizers. The variable retarders are driven such that, during a first ON or transmissive time interval, the first variable retarder receives a high voltage while the second variable retarder receives zero volts and, during a second OFF or blocked time interval, both first and second variable retarders receive high voltages. The result is that the light gate turns ON to a transmissive state very quickly at the beginning of the first time interval and turns OFF to a blocked state very quickly at the beginning of the second time interval. The second time interval is followed by a third time interval of indefinite duration during which both variable retarders receive zero volts and relax to their unpowered states. The light gate is in the blocked state during the third time interval. This relaxation is comparatively slow during the third time interval because it is unpowered and must be completed before the light gate can be reactivated. This scheme is unsuitable for time-multiplexed stereoscopic 3D applications, which operate with two time intervals (left and right image subframes) of substantially equal durations.
Bos in U.S. Pat. No. 4,719,507 describes a time-multiplexed stereoscopic imaging system embodiment comprising a linear polarizer and first and second liquid crystal variable optical retarders whose optic axes are perpendicular to each other. The variable retarders are separately switched such that, during a first image frame, the first variable retarder is in a zero retardation state and the second variable retarder is in a quarter-wave retardation state resulting in right circularly polarized light and, during a second image frame, the first variable retarder is in a quarter-wave retardation state and the second variable retarder is in a zero retardation state resulting in left circularly polarized light. At no time does the second variable retarder compensate the change the first variable retarder makes to the input polarization state of incident light. During switching, one variable retarder is powered on while the other variable retarder is simultaneously powered off and vice versa. A disadvantage of this scheme is that both transitions incorporate the comparatively slow unpowered transition, which can introduce image crosstalk, loss of brightness, and other artifacts in modern time-multiplexed stereoscopic 3D applications.
Cowan et. al. in U.S. Pat. No. 7,477,206, describe a polarization switch, which in a manner similar to that of the above-described U.S. Pat. No. 4,719,507, uses two liquid crystal variable optical retarders that are capable of switching between zero and a quarter-wave retardation and are driven in a push-pull manner. The same disadvantages of the polarization switch described in U.S. Pat. No. 4,719,507 also apply here.
Robinson and Sharp in U.S. Pat. No. 7,528,906 describe several embodiments of polarization switches that use two half-wave pi-cells optically associated in series. One embodiment uses two pi-cells constructed for surface contacting director alignment by rubbing on the surfaces of the optically transparent electrodes in a parallel direction. The two pi-cells are oriented such that the rub directions of the two pi-cells make about a 43° angle with each other. Other embodiments use two pi-cells with their rub directions parallel to each other and constructed with one or more intervening passive retardation films. In all cases, when incident light in an input polarization state propagates through the first and second pi-cells, the second pi-cell does not compensate a change that the first liquid crystal retarder makes to the input polarization state. Both liquid crystal devices are simultaneously driven with the same waveforms, resulting in a very fast optical response when both liquid crystal devices are switched from a low voltage optical state to a high voltage optical state because they are powered transitions, but the simultaneous transitions from high to low voltage optical states are unpowered transitions and are therefore comparatively slow, reducing switching performance for time multiplexing stereoscopic 3D applications.
Hörnell and Palmer in U.S. Pat. No. 5,825,441 describe a liquid crystal welding glass structure that includes two TN devices and an intervening polarizing film. At least one of the TN devices has a twist angle of less than 90°. Because of the intervening polarizer, the state of polarization of light entering the second TN device is constant, regardless of the change the first TN device makes to the input polarization state of incident light, so no compensation is involved. This arrangement gives superior performance in welding applications, in which extremely high optical density over wide viewing angles is required, but would not be suitable for time multiplexing stereoscopic 3D applications because of the slow optical response of the unpowered transitions.